ARM: 8985/1: efi/decompressor: deal with HYP mode boot gracefully

EFI on ARM only supports short descriptors, and given that it mandates
that the MMU and caches are on, it is implied that booting in HYP mode
is not supported.

However, implementations of EFI exist (i.e., U-Boot) that ignore this
requirement, which is not entirely unreasonable, given that it makes
HYP mode inaccessible to the operating system.

So let's make sure that we can deal with this condition gracefully.
We already tolerate booting the EFI stub with the caches off (even
though this violates the EFI spec as well), and so we should deal
with HYP mode boot with MMU and caches either on or off.

- When the MMU and caches are on, we can ignore the HYP stub altogether,
  since we can carry on executing at HYP. We do need to ensure that we
  disable the MMU at HYP before entering the kernel proper.

- When the MMU and caches are off, we have to drop to SVC mode so that
  we can set up the page tables using short descriptors. In this case,
  we need to install the HYP stub as usual, so that we can return to HYP
  mode before handing over to the kernel proper.

Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index c79db44..434a169 100644 (file)
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -1410,7 +1410,11 @@ memdump: mov     r12, r0
 __hyp_reentry_vectors:
                W(b)    .                       @ reset
                W(b)    .                       @ undef
+#ifdef CONFIG_EFI_STUB
+               W(b)    __enter_kernel_from_hyp @ hvc from HYP
+#else
                W(b)    .                       @ svc
+#endif
                W(b)    .                       @ pabort
                W(b)    .                       @ dabort
                W(b)    __enter_kernel          @ hyp
@@ -1429,14 +1433,72 @@ __enter_kernel:
 reloc_code_end:
 
 #ifdef CONFIG_EFI_STUB
+__enter_kernel_from_hyp:
+               mrc     p15, 4, r0, c1, c0, 0   @ read HSCTLR
+               bic     r0, r0, #0x5            @ disable MMU and caches
+               mcr     p15, 4, r0, c1, c0, 0   @ write HSCTLR
+               isb
+               b       __enter_kernel
+
 ENTRY(efi_enter_kernel)
                mov     r4, r0                  @ preserve image base
                mov     r8, r1                  @ preserve DT pointer
 
+ ARM(          adrl    r0, call_cache_fn       )
+ THUMB(                adr     r0, call_cache_fn       )
+               adr     r1, 0f                  @ clean the region of code we
+               bl      cache_clean_flush       @ may run with the MMU off
+
+#ifdef CONFIG_ARM_VIRT_EXT
+               @
+               @ The EFI spec does not support booting on ARM in HYP mode,
+               @ since it mandates that the MMU and caches are on, with all
+               @ 32-bit addressable DRAM mapped 1:1 using short descriptors.
+               @
+               @ While the EDK2 reference implementation adheres to this,
+               @ U-Boot might decide to enter the EFI stub in HYP mode
+               @ anyway, with the MMU and caches either on or off.
+               @
+               mrs     r0, cpsr                @ get the current mode
+               msr     spsr_cxsf, r0           @ record boot mode
+               and     r0, r0, #MODE_MASK      @ are we running in HYP mode?
+               cmp     r0, #HYP_MODE
+               bne     .Lefi_svc
+
+               mrc     p15, 4, r1, c1, c0, 0   @ read HSCTLR
+               tst     r1, #0x1                @ MMU enabled at HYP?
+               beq     1f
+
+               @
+               @ When running in HYP mode with the caches on, we're better
+               @ off just carrying on using the cached 1:1 mapping that the
+               @ firmware provided. Set up the HYP vectors so HVC instructions
+               @ issued from HYP mode take us to the correct handler code. We
+               @ will disable the MMU before jumping to the kernel proper.
+               @
+               adr     r0, __hyp_reentry_vectors
+               mcr     p15, 4, r0, c12, c0, 0  @ set HYP vector base (HVBAR)
+               isb
+               b       .Lefi_hyp
+
+               @
+               @ When running in HYP mode with the caches off, we need to drop
+               @ into SVC mode now, and let the decompressor set up its cached
+               @ 1:1 mapping as usual.
+               @
+1:             mov     r9, r4                  @ preserve image base
+               bl      __hyp_stub_install      @ install HYP stub vectors
+               safe_svcmode_maskall    r1      @ drop to SVC mode
+               msr     spsr_cxsf, r0           @ record boot mode
+               orr     r4, r9, #1              @ restore image base and set LSB
+               b       .Lefi_hyp
+.Lefi_svc:
+#endif
                mrc     p15, 0, r0, c1, c0, 0   @ read SCTLR
                tst     r0, #0x1                @ MMU enabled?
                orreq   r4, r4, #1              @ set LSB if not
 
+.Lefi_hyp:
                mov     r0, r8                  @ DT start
                add     r1, r8, r2              @ DT end
                bl      cache_clean_flush